Electrographic recording elements, also known as electrographic recording materials, electrostatic imaging materials, or dielectric recording materials, comprise a conductive base and a dielectric coating. In use, an electrical charge pattern is applied to the dielectric coating, by, for example, an array of styli or other electrodes. To produce an image, this charge pattern is made visible by the application of a toner, normally in the form of a dry powder or a non-aqueous dispersion. To form a permanent image, the toned image is fixed by, for example, heating or by removal of the solvent.
Paper, polymeric film, or other sheet material may be used as the base for the dielectric coating. The base is normally rendered conductive by means of a electroconductive composition, which may be applied at the size bath, in the case of paper, or by other coating means. Salts, most usually polymeric quaternary ammonium compounds, such as are described in Schaper et al. U.S. Pat. No. 3,486,932, have been used as electroconductive compositions
In a typical conventional electrostatic recording element, a dielectric layer is formed on a base covered with an electroconductive layer with a surface-specific resistivity of 10.sup.5 to 10.sup.9 ohms. The surface-specific resistivity of the electroconductive layer, may, however, drop below 10.sup.5 ohms, or exceed 10.sup.9 ohms, depending on the humidity. Due to the effects of humidity, the optical density of the image may decrease, or, in extreme cases, recording may become totally impossible.
Humidity sensitivity is due to the nature of the electroconductivity of the electroconductive composition. For salts, such as polymeric quaternary ammonium compounds, the electroconductivity of the electroconductive composition is due to ionic conduction. Therefore, the resistivity of the element is affected by its water content. When the electroconductive base is left in low humidity for a long time, its water content decreases, causing the ionizing capacity to deteriorate with a resultant increase in resistivity. If the dielectric recording element is left in high humidity, its water content increases with a resultant decrease in resistivity.
To overcome this problem, non-ionic conductive fillers, for example: powders of metals such as nickel, copper, and aluminum; silver powder; carbon black; conductive fibers; copper iodide powder; and synthetic hectorite clays, have also been used in electroconductive compositions. However, oxide films are produced on the surfaces of metal powders increasing their contact resistance; silver powder is expensive; and carbon black, conductive fibers, copper iodide powder, and clays can impart unwanted color and opacity to the electrographic recording element.
Thus, a need exists for an electroconductive composition containing a non-ionic electroconductive powder which has a high and stable electroconductivity; is relatively insensitive to humidity; is inexpensive; has uniform properties, such as particle size and composition; and does not impart undesirable color and opacity to electrographic recording elements.